.main,
.left,
.right {
    float: left;
    height: 200px;
    text-align: center;
}

.main {
    width: 100%;
    background: #ace;
}

.left {
    width: 20%;
    background: #eee;
    margin-left: -100%;
}

.right {
    width: 20%;
    background: #ddd;
    margin-left: -20%;
}